This invention relates to sand screens for petroleum wells, and especially to the protection of sand screens from erosion.
Oil and gas are produced from underground formations through wellbores drilled from the surface to the formation. Some oil and gas are contained in the pores of consolidated rocks. When this petroleum (oil, gas, or both) is produced through the wellbore, it is generally relatively free of sand. However, many oil and gas wells produce fluid from underground formations which are friable or unconsolidated; that is, the sand particles are not strongly attached to each other, and, when the petroleum containing fluid (possibly with substantial amounts of water) is produced, it tends to carry entrained sand with it. This sand can cause extremely serious damage to well equipment. In producing petroleum from such unconsolidated formations, some method should be provided to restrain the sand inflow into the well.
Various configurations of sand screens have been used. U.S. Pat. No. 3,712,373, issued to Bearden and Howard, on Jan. 23, 1973 (and U.S. Pat. No. 3,816,894, which is division of the aforementioned U.S. Pat. No. 3,712,373), describe multilayer (especially triple-layer) screens for use in petroleum wells in unconsolidated sand formations. The three-layer (or triple-wrap) configuration has been used extensively in fine sand formations. Unfortunately, however, erosion has caused extensive problems in these screens when used in high-production wells in the very fine sand formations. The erosion is particularly bad when the flow is concentrated in a relatively small region. This flow concentration is especially prevalent in cased holes where the flow is concentrated through the perforations. While production from the uncased section of a well (open-hole) usually avoids the problem of excessively rapid erosion of screens due to flow concentrations, irregularities in the formations occasionally cause extreme concentrations in the flow, and thus the use of open-hole sections does not always eliminate the problem. In addition, a single cased hole can be used for pretroleum production from several different formations at different depths. Thus, the use of open holes in areas with multiple producing formations requires additional wells at considerable extra expense.
Gravel packs around the outside of screens have been used, but the gravel has not proved to be an entirely satisfactory erosion-preventive method in high-production wells and is often parted by the flow stream, leaving a hole through which the sand-laden fluid flows at high velocity. The gravel may also be vibrated by the flowing fluids and thereby erode the screen. Further, gravel packing is a time-consuming operation and represents considerable added expense. Typically, two or more days of expensive rig time are required for a conventional gravel pack.
Gravel packing was used with a single-wrap screen in various combinations in five wells of the Teak Field, located about 25 miles off the east coast of Trinidad. None of these combinations proved very satisfactory in the shallower, more loosely consolidated formations, and either very low flow rates or holes in the screen (resulting in sanding up of the wells) were encountered in all wells in these formations.
Triple-wrap screens were run in eight wells without gravel packing in the same formations. While such screens appeared to be working satisfactorily when first installed, holes were formed in the relatively thin wall (about one-eighth inch per layer) of the screen in a relatively short time. This problem was especially severe in the shallower horizons and one of the triple-wrap screens failed after only 21 days of production. For further details of this installation, reference is made to "A History of Sand Control in Amoco Trinidad's Offshore Teak Field," by Don J. Likwartz, presented at the 50th Annual Fall Meeting of the Society of Petroleum Engineers to AIME, in 1975 (Paper SPE 5657).
The erosion of the screens can result in considerable expense, including the costs associated with cleaning out sand-filled lines and patching (i.e., U.S. Pat. No. 3,482,629, issued to Suman on Dec. 9, 1969), or replacing screen assemblies, as well as repair to damaged surface equipment.